Alexios V Doukas or Alexius V Ducas (Greek: Ἀλέξιος Εʹ Δούκας) was the Byzantine emperor from 5 February to 12 April 1204 during the second and final siege of Constantinople by the participants of the Fourth Crusade. He was a member of the Doukas family, nicknamed Mourtzouphlos or Murtzuphlus (Μούρτζουφλος), either in reference to bushy, overhanging eyebrows or a sullen, gloomy character.
A Byzantine nobleman, his participation in the attempted overthrow of Alexios III Angelos by John Komnenos the Fat in 1200 had led to his imprisonment until the restoration to the throne of Isaac II Angelos. Isaac II, along with his son Alexios IV Angelos, were restored to the throne through the intervention of leaders of the Fourth Crusade in July 1203. Released, Murtzuphlus was allowed the court position of protovestiarios. He had been married twice but was allegedly the lover of Eudokia Angelina, a daughter of Emperor Alexios III Angelos.
By the beginning of 1204, Isaac II and Alexios IV had inspired little confidence among the people of Constantinople in their efforts to defend the city from the Latins and Venetians, who were restless and rioted when the money and aid promised by Alexios IV was not forthcoming. Alexios Doukas emerged as a leader of the anti-Latin movement and personally led some skirmishes against the crusaders. When the populace rebelled in late January 1204, the two emperors barricaded themselves in the palace and entrusted Alexios Doukas with a mission to seek help from the crusaders. Instead, Alexios Doukas used his access to the palace to arrest the emperors. The young Alexios IV was strangled in prison, while his father Isaac died shortly afterwards, his death variously attributed to fright, sorrow, or foul play. Alexios V Doukas was crowned in early February 1204.
After his coronation, Alexios V began to strengthen the defenses of Constantinople and ended negotiations with the Latins. It was too late, however, for the new Emperor to make much of a difference. An attempted surprise attack against the crusader camp failed despite the Emperor's personal leadership. During the ensuing fight, the defenders of Constantinople held out against the crusader counterattack of 9 April. The crusaders' second attack proved too strong to repel, and Alexios V fled towards Thrace on the night of 12 April 1204, accompanied by Eudokia Angelina and her mother Euphrosyne Doukaina Kamatera. Constantinople was under Latin control by the next day.
The refugees reached Mosynopolis, the base of the deposed emperor Alexios III Angelos, where they were initially well received, with Alexios V marrying Eudokia Angelina. Later, however, Alexios III arranged for his new son-in-law to be ambushed and blinded, making him ineligible for the imperial throne. Abandoned by his supporters and enemies alike, Alexios V was captured near Mosynopolis by the advancing Latins under Thierry de Loos in November 1204. Brought back to Constantinople, Alexios V was condemned to death for treason against Alexios IV, and was thrown from the top of the Column of Theodosius. He was the last Byzantine Emperor to reign in Constantinople before the establishment of the Latin Empire, which controlled the city for the next 57 years, until it was recovered by the Nicaean Emperor Michael VIII Palaiologos.